Venlafaxine, (±)-1-[2-(Dimethylamino)-1-(4-methyoxyphenyl)ethyl]cyclo-hexanol, having the following formula I, is the first of a class of anti-depressants. Venlafaxine acts by inhibiting re-uptake of norepinephrine and serotonin, and is an alternative to the tricyclic anti-depressants and selective re-uptake inhibitors.

O-desmethylvenlafaxine, chemically named 4-[2-(dimethylamino)-1-(1-hydroxycyclohexyl)ethyl]phenol and having the following formula II
is a major metabolite of venlafaxine and has been shown to inhibit norepinephrine and serotonin uptake. Klamerus, K. J. et al., “Introduction of the Composite Parameter to the Pharmacokinetics of Venlafaxine and its Active O-Desmethyl Metabolite”, J. Clin. Phavmacol. 32:716-724 (1992).
O-desmethylvenlafaxine and processes for the preparation thereof are described in U.S. Pat. Nos. 6,197,828 and 6,689,912, and in US 2005/0197392, which are incorporated herein by reference.
The fumarate salt of O-desmethylvenlafaxine, is chemically named -[2-(dimethylamino)-1-(4-phenol) ethyl]-cyclohexanol fumarate, and has the following formula III

Several pharmaceutically acceptable salts of O-desmethylvenlafaxine are described in U.S. Pat. No. 4,535,186. In Example 26 of this reference, a preparation of the fumarate salt is described. The product is reported to have a melting point range of 140° C.-142° C.
Polymorphism, the occurrence of different crystal forms, is a property of some molecules and molecular complexes. A single molecule or molecular complex, like O-desmethylvenlafaxine fumarate, may give rise to a variety of crystalline forms having distinct crystal structures and physical properties like melting point, x-ray diffraction pattern (e.g. powder x-ray diffraction or PXRD), infrared absorption fingerprint, FTIR spectrum, and solid state NMR spectrum. One crystalline form may give rise to thermal behavior different from that of another crystalline form. Thermal behavior can be measured in the laboratory by such techniques as capillary melting point, thermogravimetric analysis (“TGA”), and differential scanning calorimetry (“DSC”), which have been used to distinguish polymorphic forms.
The difference in the physical properties of different crystalline forms results from the orientation and intermolecular interactions of adjacent molecules or complexes in the bulk solid. Accordingly, polymorphs are distinct solids sharing the same molecular formula yet having distinct advantageous physical properties compared to other crystalline forms of the same compound or complex.
One of the most important physical properties of pharmaceutical compounds is their solubility in aqueous solution, particularly their solubility in the gastric juices of a patient. For example, where absorption through the gastrointestinal tract is slow, it is often desirable for a drug that is unstable to conditions in the patient's stomach or intestine to dissolve slowly so that it does not accumulate in a deleterious environment. Different crystalline forms or polymorphs of the same pharmaceutical compounds can and reportedly do have different aqueous solubilities.
The discovery of new polymorphic forms of a pharmaceutically useful compound provides a new opportunity to improve the performance characteristics of a pharmaceutical product. It enlarges the repertoire of materials that a formulation scientist has available for designing, for example, a pharmaceutical dosage form of a drug with a targeted release profile or other desired characteristic.
There is a need in the art for polymorphic forms of O-desmethylvenlafaxine fumarate.